1. Field of the Invention
This invention relates to an elastic surface wave propagation device which can be employed as a delay line or a band-pass or band-rejection filter, and more particularly to an elastic surface wave propagation device which comprises a substrate for the propagation of elastic surface wave and at least one transducer disposed on the major surface of the substrate for converting an electric signal into an elastic surface wave or vice versa and in which the elastic surface wave derived from the transducer is propagated on the major surface of the substrate or an elastic surface wave is propagated towards the transducer.
2. Description of the Prior Art
There has heretofore been proposed an elastic surface wave propagation device of this kind which employs, as a substrate for the propagation of the elastic surface wave, a quartz crystal substrate whose major surface is a 42.75.degree. rotated Y cut plane, that is, the ST cut plane. The quartz crystal is mechanically hard. Further, in the case where a pair of electrodes are disposed side by side on the major surface of the quartz crystal substrate whose main surface is such an 42.75.degree. rotated Y cut plane, the quartz crystal substrate presents a piezoelectric effect between the pair of electrodes, and its electro-mechanical coupling coefficient is relatively large. Accordingly, only by disposing electrodes directly on the major surface of the quartz crystal substrate, a transducer of relatively high conversion efficiency can be constructed. Moreover, even if the quartz crystal substrate having its major surface formed in the ST cut plane is affected by temperature change, it does not cause any substantial change in the propagation velocity of the elastic surface wave, particularly, a Rayleigh wave which is most typical of it. Accordingly, the elastic surface wave can be effectively propagated on the major surface of the quartz crystal substrate with practically no temperature dependency.
In such a conventional elastic surface wave propagation device, the angle .theta. between the direction of propagation of the elastic surface wave on the major surface of the quartz crystal substrate and the X-axis direction of the quartz crystal is usually selected to be zero. As a result of this, the conventional device is defective in that spurious components of such magnitude as not to be negligible appear in a frequency range which is 1.7 to 1.8 times higher than the center frequency of an elastic surface wave based on a desired electric signal propagating in the quartz crystal substrate.
Thus, the conventional elastic surface wave propagation device has such advantages that it can be constructed mechanically rigid as a whole, that a transducer can be easily formed on the quartz crystal substrate and that an elastic surface wave can be propagated on the major surface of the quartz crystal without temperature dependency. However, the conventional device is defective in that spurious components of such magnitude as not to be negligible get mixed in the elastic surface wave converted from a desired electric signal.